1. Field of the Invention
The present invention relates to a power source inverter circuit, more specifically, a power source inverter circuit for electronic instruments driven by use of output of a solar module or a thermoelectric conversion element in which electric power generated is supplied to a load circuit with efficiency and excess electric power is stored in storage means.
2. Description of the Related Art
Conventional power source inverter circuits supply a load circuit with electric power of power feeding means which changes its voltage and current with time or with a change in environment and have, as shown in FIG. 12, a DC—DC converter, a rectifier element for rectifying electric power outputted from the DC—DC converter, a voltage detector for detecting the voltage of the storage means, storage means for storing rectified electric power, and a load circuit. In such conventional power source inverter circuits where a load circuit is supplied with electric power of a solar module, a thermoelectric conversion circuit, or other similar power feeding means which changes its voltage and current with time or with a change in environment, operation of a DC—DC converter is controlled by monitoring the voltage with a voltage detector such that the DC—DC converter is put into operation when electric power supplied from the power feeding means has a voltage equal to or higher than the minimum operation voltage of the DC—DC converter and that the voltage of the storage means reaches a given output voltage. For instance, if the DC—DC converter is a switched capacitor type DC—DC converter, the output voltage is kept constant by turning the DC—DC converter on and off for intermittent operation.
The electric power that has undergone voltage conversion is rectified by a rectifier element such as a Schottky diode, and then stored in the storage means through charging. At this point, the electric power stored in the storage means is supplied to the load circuit since the charging means and the load circuit are connected in parallel to each other. The load circuit is put into operation as the stored voltage of the charging means reaches the minimum drive voltage of the load circuit or higher. Accordingly, when the storage means has large capacity, the voltage of the storage means is slow to rise and activation takes extremely long. Thus prior art requires, for continuous operation of the load circuit, efficient charging of the storage means without exhausting the storage means of electric power and without allowing the voltage of the storage means to drop lower than the minimum drive voltage of the load circuit.
In addition, the step-up and step-down multiple numbers are fixed in conventional power source inverter circuits in the case of using switched capacitor type DC—DC converters as the DC—DC converters. In this case, charging cannot be conducted when there is a change in output voltage of the power feeding means and either raising or lowering the voltage cannot help the output voltage of the DC—DC converter from dropping below the output of the storage means.
Furthermore, the load circuit, which operates on electric power stored in the storage means after output of the power feeding means becomes unavailable, stops its operation once the voltage of the storage means drops below the minimum operation voltage of the load circuit. At this point, there are still electric charges left in the storage means.
Examples of the above-described means for feeding electric power that changes its voltage with time include thermoelectric conversion elements and solar panels for use in portable electronic instruments that are relatively small in power consumption. For instance, thermoelectric conversion elements, which employ PN junction between a P type semiconductor and an N type semiconductor to generate electric power from an electromotive force created by a temperature difference, change their electromotive forces (voltage) as the temperature difference changes with time.
In such conventional power source inverter circuits, supplying the load circuit with a constant voltage requires stopping the operation of the DC—DC converter or adjusting the amount of electric power taken out. The amount of electric power taken out is accordingly no larger than the amount of electric power needed by the load circuit even when the power feeding means is capable of generating and outputting more. This means that the excess electric power goes to waste. In particular, power feeding means such as a solar module or a thermoelectric conversion circuit is quick to change its output voltage in response to a change in environment or with time, thereby making it difficult to maintain the same level of output performance. Accordingly, it is necessary to take as much electric power as possible out of the power feeding means while the power feeding means has high power generation ability, namely, when the amount of light is very large or when there is a heat source.
In addition, in power feeding means that uses a natural energy source, such as a solar module or a thermoelectric conversion circuit, the relation between the output voltage and the output current has a local maximal value which equals to the maximum power generation efficiency. Since power feeding means that uses a natural energy source is small in energy amount, it is desirable to take out electric power always with the maximum power generation efficiency. Here lies another inconvenience because, in conventional power source inverter circuits where the amount of electric power to be taken out of the power feeding means is determined in accordance with power consumption of the load circuit, electric power is rarely taken out with the maximum power generation efficiency and the excess power is wasted.
Conventional power source inverter circuits, where the storage means and the load circuit are connected in parallel to the output of the rectifier element, have still another inconvenience: the storage empty of electric charges induces a voltage drop even when the output of the DC—DC converter is equal to or higher than the operation voltage of the load circuit and it is not until some electric charges are stored in the storage means that the load circuit can start its operation.
Furthermore, in conventional power source inverter circuits that use switched capacitor type DC—DC converters, the step-up multiple number or the step-down multiple number is fixed and therefore electric power cannot be stored once the output voltage of the DC—DC converter becomes lower than the voltage of the storage means irrespective of power conversion performed on the output power of the power feeding means which changes its output voltage with time or with a change in environment.
Yet another inconvenience is that the load circuit, which operates on electric power stored in the storage means when the power feeding means no longer generates power which changes its output voltage with time or with a change in environment, stops its operation as the voltage of the storage means drops below the minimum operation voltage of the load circuit. A mere voltage drop renders the load circuit ineffective even though there are enough electric power left in the storage means. Accordingly, the remaining electric power goes to waste.